Having a steep belt angle is desirable for safety reasons even for the safety belts of the rear seats in modern motor vehicles. To achieve this, the belt buckles must exit from slots in the seat in front of the rear edge of the seat. The belt buckles have been passed through hitherto by hand without a mounting aid, which is a very laborious and time-consuming operation, especially if the belt buckles are fastened to semi-rigid wire cable holders, which yield on pressure on the belt buckles.